The tubes of firetube boilers are received in holes in tube sheets and are affixed and sealed to the tube sheets by expanding the end portions of the tubes into engagement with the walls of the holes and forming beads on the ends of the tubes. Prior to performing the expanding and forming operations, the tubes are cut to length, leaving a portion of each tube extending a specified distance beyond the surface of the tube sheet. The extending portions of the tubes are formed into generally arcuate, radially outwardly extending beads having convex surfaces facing away from the tube sheet. The beads enhance the mechanical attachment between the tubes and the tube sheet and protect the ends of the tubes from flame erosion.
Tools for performing the expanding and beading operations are available. Often, one tool is used for expanding the tubes, and a reciprocating hammer is used to form the beads. Hammering tools require considerable skill to make proper beads and make a lot of noise. Forming by hammering, moreover, usually destroys the integrity of the tube-to-hole wall joints and requires expanding the tubes again after hammering.
A tool that both expands and beads the tubes of firetube boilers in a single operation is described and shown in U.S. Pat. No. 4,090,382 (Schott, 1978) and is commercially available. A mandrel having a tapered distal end receives a cage that is rotatable and movable axially relative to the mandrel and supports several expanding rolls for rotation, the rolls being engaged by the mandrel and being skewed relative to the axis of the mandrel at a very small angle. When the mandrel is driven in rotation and with the expanding rolls in engagement with the tube, the expanding rolls exert a traction force on the mandrel that pulls it distally relative to the cage, thus forcing the expanding rolls radially outward and gradually expanding the tube. The drafting action resulting from the skew of the expanding rolls also draws the cage into the tube, but only to an extent permitted by the beading roll or rolls, as described in the next paragraph.
In one embodiment disclosed in the Schott patent and in the commercially available tool, a beading roll is mounted on the cage for rotation about an axis that is radial with respect to the axis of the mandrel. The beading roll is in rolling and supporting engagement with two thrust rolls, which transmit an axial force from the cage to the beading roll (or rolls) as the cage is pulled into the tube by the distal traction of the expanding rolls. The axial force applied to the beading roll forms an arcuate bead on the end of the tube by deforming the tube end to match the shape of the forming surface of the beading roll. The formed bead acts as a stop for the cage, which stops moving into the tube when the bead is fully formed. The position of the beading roll radially of the cage and the tube is fixed--the beading roll tracks a fixed rotary path around the end of the tube. Meanwhile, the expanding rolls are translating radially outwardly relative to the axis of the tube as expanding progresses. Also, the track of the beading roll along the tube does not overlap the tracks of the expanding rolls along the tube. Accordingly, the tool of the Schott patent tends to leave a ridge projecting into the tube at the juncture between the bead and the expanded part of the tube.
The tubes of firetube boilers are of standard diameters (2", 2.5" and 3"), but each size comes in different gages. In order to match the bead to the gage of the tube, the tool of the Schott patent requires a different beading roll for each gage, thus requiring the manufacturer to stock different beading rolls and to replace the beading roll of each tube each time a firetube boiler with a different gage tube is being fabricated.
The beading roll is subject to very large axial forces and very high contact stresses. The contact stresses are particularly high between the beading roll and each of the two thrust rolls, which make essentially line contact between convex surfaces of small radii. The high contact stresses result in wear and a short life for the beading roll and the thrust rolls.